Have you checked the throttle linkage to make sure you're really getting full throttle?

On my 300D, I found a while back that there was still 1/8" of arm travel at the injector pump with the pedal to the floor. I adjusted the linkage, and it was like a completely new car. I couldn't believe it...

Also, at least on the OM617, there's a plastic hose that goes between the intake manifold and the ALDA on the injector pump. This tells the ALDA how much boost pressure there is, and the ALDA then has the injector pump add fuel accordingly. If this plastic hose is gunked up (common, unfortunately, courtesy of the EGR valve), you can have power loss issues...also, on this line there's a 3-way electric valve, so that if a sensor on the intake manifold senses too much boost, the boost signal going to the pump is instead vented to thin air.

I don't know how much either of these will help with an OM603, but I hope it at least gives you some ideas on things to check. Good luck!